The invention relates to the technical field of assembling sheets without welding.
Various methods are already known in the prior art for assembling sheets without welding.
Of those methods, particular mention can be made of embedding, stapling, riveting, adhesive, bolting, and crimping.
The present invention relates more particularly to assembling sheets by crimping.
The term xe2x80x9ccrimpingxe2x80x9d is used to mean mechanical assembly in which at least one of the members is locally deformed in permanent manner.
Crimping makes it possible to assemble together a wide variety of materials that are not usually assembled together such as, for example, plastics materials, leather, glass, wood, copper alloy, aluminum alloy.
The invention relates more particularly, but not exclusively, to assembling together metal or metal alloy sheets by crimping.
Various crimping methods of that type are already known in the prior art.
By way of example, reference can be made to the following documents: EP 0 383 993; FR-A-2 565 871; and FR-A-2 371 252.
Document FR-A-2 371 252 describes a method of assembling together two pieces which are in contact via a surface that is plane or that has a small amount of curvature that is regular, at least one of the pieces being a plate or a sheet of a ductile material such as a metal.
In that known prior art method, a bore having two successive diameters is provided in one of the pieces, the smaller diameter being on the side that is in contact with the plate or sheet to be assembled with said piece, said plate or sheet being provided with a bore of diameter that is smaller than the smaller of the diameters in the bore of the piece.
After placing the two bores so that they are in axial alignment, the metal situated in the vicinity of the bore in the plate or the sheet is driven into the bore of the piece to be fixed and at least as far as the larger portion of said bore.
The method described in document FR-A-2 371 252 is more particularly intended for fixing a reinforcing washer onto a sheet of mild steel.
The washer is fixed in place by driving metal of the washer into the bore in the sheet.
The assembly method described in document FR-A-2 371 252 is very suitable when the assembly formed by the washer, the sheet, the backing die, and the punch is in accurate axial alignment, however if any one of those elements is not properly on the axis relative to the others, then the fixing that is obtained will be faulty both functionally and in terms of appearance.
More precisely, in the apparatus described in document FR-A-2 371 252, when there is an axial offset of about one-tenth of a millimeter between the washer and the sheet, then the consequences can be as follows:
the punch stretches the metal of the washer more on one side of its bore than on the other;
in the portion where the metal of the washer is more greatly stretched, the volume of material that is displaced can be greater than the design volume provided in the bore in the sheet;
the punch which has a cylindrical portion for calibrating the diameter of the final hole will push the surplus material into the die piece and cut off said surplus material, giving rise to an irreparable and unacceptable burr caused by the operating clearance between the die and the punch; and
conversely, in the portion where the metal is deformed insufficiently, the volume of displaced material is smaller than the design volume provided in the bore in the sheet and insufficient for completely filling the bottom portion of the bore. This defect reduces tear-out strength.
A first object of the invention is to provide a crimping assembly method of the buttoning type that leads to no loss of tear-out strength between the two assembled-together parts in the event of said assembly being performed in the presence of an alignment fault.
A second object of the invention is to provide a method of assembling together two sheets that does not require an additional piece and that makes it possible with a small number of operations to obtain accurate fixing of the two pieces relative to each other while guaranteeing that the fixing system proper can accommodate the clearance necessary for accurate positioning of the two pieces to be assembled together without the fixing suffering as a result from poor tear-out strength or poor appearance.
To this end, in a first aspect, the invention provides a method of assembling together, without welding, first and second pieces of plastically deformable material such as a ductile metal or metal alloy, said method comprising:
in a first step, making a rim hole in the first piece, the hole having a rim with an inside diameter D2i and an outside diameter D2e, and extending along an axis substantially perpendicular to the local mean plane of the first piece;
a step of making a counterbored hole having two successive diameters in the second piece, said counterbored hole extending along an axis substantially perpendicular to the local mean plane of the second piece, said counterbored hole having a first section of small diameter d8 and a second section of large diameter D8, the small diameter d8 being greater than the outside diameter D2e of the rim by a xe2x80x9cclearancexe2x80x9d value;
a step of inserting said rim into said counterbored hole, an annular gap being defined between the outer annular face of the rim and the inner annular face of the second section of the counterbored hole; and
a step of deforming the end portion of the rim so as to occupy at least part of the annular gap;
said method comprising prior to the step of deforming the end portion of the rim, a step of inserting centering means or xe2x80x9ccenter punchxe2x80x9d in the rim hole, the centering means having a diameter Dc equivalent to that of the first section of the counterbored hole minus twice the thickness of the rim, i.e.:
Dc=d8xe2x88x92(D2exe2x88x92D2i)
the axis of the centering means or being maintained in a predefined reference position.
According to a particular characteristic, the method is such that the dimensions of the rim hole and of the counterbored hole are predetermined so that the portion of the rim which is pressed into the counterbore:
always bears against the counterbore;
always extends beyond the small diameter d8 of the first section of the counterbored hole; and
does not extend beyond the larger diameter D8 of the second section of the counterbored hole; even when the axes of the rim hole and of the counterbored hole are in their position of maximum offset.
In various implementations, the present method includes the following characteristics, taken singly or in combination:
the step of inserting the centering means is performed after holding the first piece in position relative to the second piece;
the first piece and the second piece are held in contact during insertion of the centering means via respective substantially plane surfaces disposed around the rim hole and the counterbored hole;
the first piece and the second piece are held in contact during insertion of the centering means via respective surfaces having a small amount of regular curvature disposed around the rim hole and the counterbored hole;
the rim hole is deformed by cold deformation;
the counterbored hole is obtained by cold deformation;
the counterbored hole is circular; and
the counterbored hole is oblong.